1. Field of the Invention
The present invention relates to an alkaline battery separator, i.e., a separator for an alkaline battery, and a process for producing the same.
2. Description of the Related Art
In an alkaline battery, a separator is used between a positive electrode and a negative electrode to separate them and to prevent a short circuit therebetween, and further, to hold an electrolyte thereon and enable a smooth electromotive reaction.
Recently, the space allotted for the battery has become smaller in electronic equipment, due to the need for miniaturization and weight-saving. Nevertheless, the performance requirement for such a smaller battery is the same as or higher than that for a conventional battery, and therefore, it is necessary to increase the capacity of the battery, and to increase the amounts of active materials in the electrodes. Thus, the volume allotted in the battery for the separator must be reduced, and the thickness of the separator must be made thinner. However, if a conventional separator is simply thinned, the capacity thereof for holding an electrolyte (i.e., the electrolyte-holding capacity) is lowered, and a dispersibility of fibers is liable to become lower. Therefore, Japanese Unexamined Patent Publication (Kokai) No. 7-29561 and Japanese Unexamined Patent Publication (Kokai) No. 8-138645, for example, disclose that the electrolyte-holding capacity and the dispersibility of fibers can be enhanced by forming a fiber web by a wet-laid method, using dividable composite fibers capable of producing fine fibers having a linear density of 60 xcexcg/m or less. The separators disclosed in these Japanese Publications have an advantageous electrolyte-holding capacity and dispersibility of fibers, but are liable to be broken by the tension generated during a step of assembling electrodes during battery production. Further, an electrode flash and an active material may break and penetrate the separator, to thus cause a short circuit between the electrodes, and therefore, the yield is poor. Further, the separators have low tear-strength and low bending resistance which also lowers the yield.
Accordingly, the object of the present invention is to remedy the above disadvantages of the prior art, and to provide an alkaline battery separator having excellent electrolyte-holding capacity, tensile strength, tear strength, and bending resistance, and ability to be used to stably produce a battery, and wherein an electrode flash rarely breaks the separator to cause a short circuit between the electrodes.
Other objects and advantages of the present invention will be apparent from the following description.
In accordance with the present invention, there is provided an alkaline battery separator comprising a nonwoven fabric containing one or more mixture layers of entangled short fibers and entangled long fibers, wherein the fiber length of the short fibers is from 1 mm to less than 25 mm, the fiber length of the long fibers is 25 mm or more, and the total thickness of all of the mixture layers accounts for not less than one-third of the whole thickness of the nonwoven fabric.
Further, in accordance with the present invention, there is provided a process for producing an alkaline battery separator comprising the steps of laminating at least one fiber web of short fibers and at least one fiber web of long fibers, a fiber length of the short fibers being from 1 mm to less than 25 mm, and the fiber length of the long fibers being 25 mm or more; and entangling the resulting laminate to form one or more mixture layers of short fibers of the fiber web of short fibers entangled with long fibers of the fiber web of long fibers so that the total thickness of all of the mixture layers accounts for not less than one-third of a whole thickness of the nonwoven fabric.